The season of the falling stars started and I want to try to achieve some picture of stars and falling one too.
I see a lot of picture with a immense number of stars but they are still. Now I'm concern about the shutter speed.
What is the amount of time at a given aperture to get still bright stars without trails?
And what are your tips for taking good picture?
I need to know because I'm not sure if the 30" of my EOS are enough or I need to but a remote bulb.

4 Answers
4

Well, the gist is - it depends on how picky you are about trails. It will almost always start to trail immediately, but it may not be noticeable until a certain point.

Additionally, forget normal exposure rules for astrophotography. It's generally about getting the most light in that you can. The odds of overexposure are pretty slim unless you're in a very light polluted area.

First off, your aperture will have pretty nothing to do with the amount of time before you get star trails. Its strictly a function of the rotation of the earth and the perceived trails is a function of your exposure time vs the focal length. The shorter the length, the longer you can track without noticeable trails.

If you go ultrawide and don't really be too picky, folks have been known to go up to 90 seconds.

At 50mm, you're really only talking about 18 seconds or so before they're noticeable (based on my own experience).

At 300mm, you can get away with at most 5 seconds before trailing shows up.

These times will vary somewhat, as the apparent rotational movement of the sky varies between the poles and the equator.

If you want to track longer, you're going to need a tracking mount of some kind that follows the sidereal rotation. You can get a dedicated unit like an Astrotrac, you can piggy back on a telescope that already does tracking, or you can build a barndoor mount. See this question for more ideas there (stacking multiple images is covered there as well).

Other tips:

You'll want the darkest sky you can imagine and make sure an point it away from the nearest city. That city 'glow' will show up in your pictures as light pollution.

You'll want a large aperture - that way you can get more light in the short amount of time you have, but pick a sharp one. So if you've got a 50mm 1.8, you may want to stop down to 2.8 to increase the sharpness.

Your next big challenge is focus. Its incredibly difficult to focus on distant stars. You have a few options, but your best choice is a Bahitov mask - mask in front of your lens that produces a particular diffraction pattern to help with focus. Check this question for more details.

I use the "Rule of 600". Divide your focal length into 600, and the result is your maximum exposure without the stars trailing. So with a 50mm lens, you can have a 12 second exposure. With a 15mm lens, a 40 second exposure.

You generally want as wide an angle as possible, between 10-20mm if you have it. 30 seconds is fine if you bump the ISO up to say 1600.

If your camera has a long exposure noise reduction, use it.

Use a lens hood to block stray light.

Focus is difficult. You may want to prefocus in daylight and take note of where infinity is - hopefully you can use a flashlight and manually focus to that point (or use gaffer tape to lock it down)

Long exposure noise reduction doesn't work well for most wide field AP work at all. In fact, you really want to make sure that you disable it and take dark frames separately. Unless you're doing a single light frame. Going as wide as possible is completely dependent on what you want to do. Bumping ISO up is not so cut and dry, especially esp your DR typically falls as your ISO increases. If you prefocus in daylight and the nighttime temp is considerably different your infinity focus may change.
–
rfuscaAug 8 '11 at 23:19

I guess it's camera dependent. I use ISO 2000 and long exposure NR and it works beautifully.
–
MikeWAug 9 '11 at 3:32

If you're stacking at all, then LE NR severely decreases the number of frames you can stack before your subject rotates out of frame.
–
rfuscaAug 9 '11 at 3:36

You need a fairly long shutter, 30 seconds+ but the problem is that you will quickly run into tracking issues. Unless you have a system set up to counteract the earth's rotation, you will begin to move within your frame.

There are lots of systems out there, here's one example, but the basic idea is just a motorized camera mount that moved opposite to the earth's rotation, allowing you to take long exposures of stars.